A check valve can be located in an intake passage leading to a cylinder in an engine to obstruct back-flow of fluid out of the cylinder into the intake passage. In such an arrangement, a downstream portion is defined by the portion of the intake passage between the check valve and cylinder.
A secondary passage can be connected to a bypass port formed on the intake passage between the check valve and cylinder to allow communication between the downstream portion and a secondary fluid source. Such communication can improve engine performance if properly controlled.
For example, such a secondary passage can enable a secondary air flow into the downstream portion. This can result in a reduction in the air flow which must pass through the check valve since a portion of the air flow can bypass the check valve and flow into the cylinder via the secondary passage and downstream portion. The reduction in pressure of the fluid flow caused by its passage through the check valve can therefore be reduced since the pressure reduction is proportional to the quantity of fluid flow through the check valve.
Reducing the fluid flow through the check valve can also result in a reduction in the required size of the check valve. A reduction in the check valve size can result in improved durability and frequency response of the check valve. Also, a reduction in the check valve size can reduce the space required in the engine to mount the check valve. This can allow the check valve to be mounted closer to the cylinder thereby reducing the size of the downstream portion.
Such a secondary passage can, however, allow a portion of the fluid which flows back into the downstream portion from the cylinder to bypass the check valve and exit the downstream portion via the secondary passage. The obstruction provided by the check valve to fluid flow from the cylinder back into the downstream portion is thereby limited.